High sensitivity, solid state neutron detector

ABSTRACT

An apparatus ( 200 ) for detecting slow or thermal neutrons ( 160 ). The apparatus ( 200 ) includes an alpha particle-detecting layer ( 240 ) that is a hydrogenated amorphous silicon p-i-n diode structure. The apparatus includes a bottom metal contact ( 220 ) and a top metal contact ( 250 ) with the diode structure ( 240 ) positioned between the two contacts ( 220, 250 ) to facilitate detection of alpha particles ( 170 ). The apparatus ( 200 ) includes a neutron conversion layer ( 230 ) formed of a material containing boron-10 isotopes. The top contact ( 250 ) is pixilated with each contact pixel extending to or proximate to an edge of the apparatus to facilitate electrical contacting. The contact pixels have elongated bodies to allow them to extend across the apparatus surface ( 242 ) with each pixel having a small surface area to match capacitance based upon a current spike detecting circuit or amplifier connected to each pixel. The neutron conversion layer ( 860 ) may be deposited on the contact pixels ( 830 ) such as with use of inkjet printing of nanoparticle ink.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation Application of, and claims benefit ofand priority to, U.S. patent application Ser. No. 13/146,780, entitled“HIGH SENSITIVITY, SOLID STATE NEUTRON DETECTOR” filed on Jul. 28, 2011,which is a 371 National Stage Application of PCT/US2009/032557, entitled“HIGH SENSITIVITY, SOLID STATE NEUTRON DETECTOR” filed on Jan. 30, 2009,which are each incorporated herein by reference in their entirety.

CONTRACTUAL ORIGIN

The United States Government has rights in this invention under ContractNo. DE-AC36-08GO28308 between the United States Department of Energy andthe Alliance for Sustainable Energy, LLC, the manager and operator ofthe National Renewable Energy Laboratory.

BACKGROUND

Neutron detection is used for a variety of purposes. For example,neutron detectors are used to enhance safety in nuclear power facilitiesbecause neutron radiation can be a safety hazard with use of nuclearreactors. With the development of nuclear weapons, there has been anincreasing need for high sensitivity neutron detectors to safeguardnuclear materials and weapons, to verify treaty and regulationscompliance, and to recover military payloads. Significantly, neutrondetectors are needed to minimize the risk of nuclear weaponsproliferation. Many are concerned that weapons grade plutonium or otherradioactive materials may be stolen and transferred across countryborders for use by terrorists or warring factions or countries. Neutrondetectors may be used at ports of entry such as harbors, bordercrossings, and airports to detect the presence of radioactive materialssuch as plutonium that release neutrons as neutrons cannot be easilyhidden with shielding. Such neutron surveillance must be accomplishedwithout undue restriction or disruption of traffic flow and events.

Unfortunately, neutron detection is not an easy science, and developersof neutron detectors face a number of difficult challenges. A naturallyoccurring neutron fluence is always present, and this fluence varieswith the molecular composition of adjacent soil, water, buildings, andso on as well as with latitude and elevation. The time variance of thebackground fluence has been described as having a Poisson distributionwith respect to time. Thus, the extraction of meaningful data has torely on obtaining sufficient data to make statistically meaningfulconclusions. Typically, when searching for contraband neutron sources,the neutron flux emission is very low and not readily separated from thebackground signature; thus, large detectors can collect data morerapidly than smaller ones. Another challenge in designing a neutrondetector is that neutrons are electrically neutral, do not respond toelectric fields, and are weakly interacting with electrons. Hence,neutrons do not ionize atoms except by direct collision with nuclei offew selected element isotopes, which makes conventional gaseousionization detectors ineffective in neutron detection.

Presently, costly and bulky pressurized tubes using rare Helium-3 gasare used to detect neutrons. These conventional neutron detectors areconsidered to be within a class of conventional neutron detectorslabeled gas-filled counters that require the application of high voltageand gamma rejection circuitry. In practice, the Helium-3 filled tubesalso require careful handling since they can indicate false positiveswhen abruptly moved or struck (e.g., provide an undesirable microphonicresponse). These types of conventional neutron detectors are effectivein many types of field operations, but they are not suitable foroperations requiring compact (e.g., covert) and highly sensitive devicescapable of functioning for long periods of time with low powerconsumption. In addition, these types of detectors are typicallyhand-fabricated and use Helium-3 gas that is generated in a nuclearreactor, making them expensive to manufacture in any quantity. The highcost of these devices has severely limited their deployment in areassuch as border crossings, cargo container inspection equipment, and thelike where they could be used to detect movement of contraband such asplutonium or plutonium-based weapons.

In some attempts to create an improved neutron detector, someresearchers have used solid-state electronics to sense alpha particlesemitted from a neutron converter material in which a reaction has takenplace in which a neutron has collided and generated one or more alphaparticles. The role of the converter material is to convert incidentneutrons into emitted charged particles, which are more readily sensed.When the emitted charged particle transits a semiconductor device, itliberates bound charges in its wake, and these charges may be collectedand used to sense the event stimulated by the initial neutron reaction.Such devices therefore serve as neutron detectors including convertermaterial and a semiconductor-based detector. For example, a boron-10 andlithium-6 metal, e.g., a neutron detection layer, has been applieddirectly to a crystalline device (such as a gallium arsenide crystallinePIN diode) to provide a neutron detector. However, the use ofcrystalline diode structures in neutron detectors has its own set ofdrawbacks and limitations. The internal noise level of an uncooledcrystalline diode is appreciable, and consequently researchers havefound it difficult to measure low background levels of ambient thermalneutrons in the surrounding area or to detect single neutron eventsusing diodes of any consequential size. A typical crystalline diode alsohas a thick semiconductor layer in which charges are collected, and itcan be expensive and difficult to grow large crystalline detectors.Charges liberated by gamma rays are also collected in the thicksemiconductor layer, and these charges contribute to the non-neutronnoise signal of the detector. It is imperative that gammas not bemistaken for neutrons since in a typical environment the backgroundgamma fluence greatly exceeds the neutron fluence expected from atypical source of neutrons.

The drawbacks associated with such solid-state neutron detectorsincluding high cost, small size, and difficulty to manufacture haveresulted in continued use of the bulky and expensive Helium-3pressurized tube devices to detect neutrons. There remains a need for ahigh sensitivity neutron detector that can be more easily manufactured,that is less expensive (e.g., allowing neutron detectors to be morewidely implemented and used), and that can be readily scaled in size(e.g., monitor a larger surface area to support detection of radioactivematerials such as smuggled plutonium and plutonium-based weapons hiddenin moving objects such as objects on conveyor belts and the like).

The foregoing examples of the related art and limitations relatedtherewith are intended to be illustrative and not exclusive. Otherlimitations of the related art will become apparent to those of skill inthe art upon a reading of the specification and a study of the drawings.

SUMMARY

The following embodiments and aspects thereof are described andillustrated in conjunction with systems, tools and methods that aremeant to be exemplary and illustrative, not limiting in scope. Invarious embodiments, one or more of the above-described problems havebeen reduced or eliminated, while other embodiments are directed toother improvements.

Briefly, neutron detectors are described that are all thin-film. Forexample, a neutron detector is presented that includes a thin film ofboron-10 containing materials such as may be deposited using thin-filmdeposition techniques such as sputtering or evaporation or by spraying,inkjet printing, or the like an ink that may include boron-10 (orlithium-6) or nano- or microparticles of boron-10 (or lithium-6)containing materials (such as boron-10 carbide, nitride, boron, or thelike). The neutron detector further may include a thin-film detectinglayer that detects alpha particles emitted from the neutron conversionlayer, and some embodiments utilize an amorphous silicon-based p-i-ndiode structure with a bottom and top contact layer. In some cases, thetop contact is a pixilated layer of metal such as palladium, with eachof the pixels being relatively long, thin strips such that they canextend to the edge of the detector for contacting while providinglimited capacitance to facilitate single neutron event detection byobserving the current spike produced in the amorphous silicon diode by aconnected amplifier or the like. For example, some detectors may includerectangular pixels arranged in a parallel manner on the detector andwith surface areas of less than about 1 cm² and more typically of lessthan about 0.5 cm² to limit capacitance.

The neutron conversion layer may be relatively thin, such as less thanabout 5 microns and more than about 1 micron and typically about 2microns, to facilitate passing of alpha particles with detectable energyto the detecting layer. This may provide only limited efficiency ofneutron detection (e.g., about 3 percent or less). Some neutrondetection devices may increase this efficiency or device sensitivity byproviding additional neutron conversion layers (e.g., sandwich thedetecting layer between two conversion layers) or provide two to ten ormore neutron detectors stacked on top of each other. In this manner,detection devices can readily be provided with sensitivities in the 10to 50 percent range. The detection devices may also be used in neutronimaging applications such as where the pixel size could be reducedaccording to spatial resolution requirements and the resultant read outin a video frame format (as one non-limiting example).

More particularly, an apparatus is provided for detecting slow orthermal neutrons. The apparatus includes an alpha particle-detectinglayer that is formed of a thin film of silicon material. In someembodiments, the detecting layer is a hydrogenated amorphous siliconp-i-n diode structure, and the apparatus includes a bottom metal contactand a top metal contact with the diode structure positioned between thetwo contacts to facilitate detection of charged particles transiting thelayer. In other embodiments, the detecting layer may be an n-i-pamorphous silicon diode structure. The amorphous silicon detector mayflow a very low reverse current under applied reverse bias such as 2 to20 volts, so that the current produced by the alpha particle transit iseasily detected over the reverse current of the diode. The apparatusfurther includes a neutron conversion layer including material with orenriched with boron-10 isotopes, which emit alpha particles that strikethe detecting layer when neutrons are received by or strike the neutronconversion layer. The top contact may be a pixilated metal layer witheach contact pixel extending to or proximate to an edge of the apparatus(e.g., an edge of a supporting layer or substrate) to facilitateelectrical connections/contacting. The contact pixels may have elongatedbodies to allow them to extend across the apparatus surface and eachbody of a pixel may have a surface area of less than about 10 cm² andmore than about 0.5 cm². In some embodiments, the neutron conversionlayer is deposited on top of these contact pixels of the top contact,which results in the conversion layer also including a plurality ofpixels. In other cases, the neutron conversion layer is electricallyinsulating so it can be deposited on top of the pixels as a single layerwithout a plurality of pixels. In other cases, the plurality of metalcontacts to the amorphous silicon detector may be deposited before theamorphous silicon and the top contact may be a single metal layer. Inother cases, the apparatus may include a substrate and the neutronconversion layer may be positioned between the substrate and thedetecting layer. In other cases, the apparatus may include two neutronconversion layers and the detecting layer may be sandwiched betweenthese two boron-10 containing layers, which may have a thickness of lessthan about 2 microns (e.g., 1 to 1.5 microns or the like) to betterassure alpha particles are delivered to the detecting layer. Thisconcept can be extended to include a continuous process whereby layerupon layer of boron and p-i-n or n-i-p diodes can be deposited to form amultilayer sandwiched detector in a continuous or repeated manufacturingprocess.

In some cases, a neutron detecting device is provided with the neutronconverting layer deposited on a thin substrate and the amorphous siliconPIN or NIP diode detecting layer deposited on a separate substrate. Insuch an embodiment, the deposited neutron conversion layers and PIN orNIP diode detecting layers may be oriented such that they are facingeach other in abutting contact or separated by a thin gap of air orother material through which the alpha particles generated by neutronscan easily travel to the PIN or NIP diode detecting layer.

In addition to the exemplary aspects and embodiments described above,further aspects and embodiments will become apparent by reference to thedrawings and by study of the following descriptions.

BRIEF DESCRIPTION OF THE DETAILED DRAWINGS

FIG. 1 is a schematic view of a neutron detector or neutron detectiondevice showing use of a neutron conversion layer (e.g., a thin layer ofmaterial including B-10) in combination with a thin film detecting layer(e.g., amorphous silicon or the like useful for detecting alphaparticles emitted from the neutron conversion layer);

FIG. 2 is a sectional view of an embodiment of a neutron detectorshowing use of pixilated top electrodes or contacts along with anamorphous silicon thin film as the detecting layer;

FIG. 3 is a top view of the neutron detector further showing details ofthe pixilated top electrodes including their elongated or narrow-bodydesign allowing them to extend to the edge or side of the detector (orsupporting substrate/layer) to facilitate electrical connection of thedetector;

FIG. 4 is a sectional view similar to that of FIG. 2 showing anotherembodiment of a thin film neutron detector;

FIG. 5 is a sectional or end view similar to those of FIGS. 2 and 4illustrating a neutron detection device that includes a number orplurality of thin film neutron detectors or sheets/layers in a stackedarrangement to enhance neutron detection efficiency (e.g., betterdetecting neutrons that pass through multiple detectors or layers ratherthan through a single neutron conversion layer);

FIG. 6 is a top view similar to that of FIG. 3 showing a neutrondetection device that includes two or more stacked neutron detectors orlayers rotated such that the elongated, pixilated top electrodes arearranged at angular offsets (e.g., are not parallel to each other and,in some embodiments, with their longitudinal axes being orthogonal toeach other);

FIG. 7 shows one embodiment of a neutron detection device illustratingthe electrical connection of a neutron detector to a number of pulsedetection components (e.g., pulse detection amplifier circuits orchip-based detectors) and also illustrating another configuration ofpixilated top electrodes or contacts;

FIGS. 8A-8D illustrate sectional views of a thin film neutron detectorduring several manufacturing or fabrication processes resulting in theapplication of a neutron conversion layer (pixilated in this example)over pixilated top contacts; and

FIG. 9 illustrates fabrication of a neutron detector with separatefabrication of the PIN diode/detector and of the neutron conversionlayer/film and later assembly of these components or assemblies toproduce a neutron detector.

DESCRIPTION

Briefly, the following description is directed to enhanced neutrondetectors and methods of manufacturing high sensitivity neutrondetectors. The inventors recognized that neutron detectors could beproduced using thin film manufacturing techniques to provide thin filmsto provide both the neutron conversion layer and the detecting layer.The neutron conversion layer can be fabricated according to the teachingherein to control cracking and peeling of this layer, which includes theisotope boron-10 (B-10 or 10B) or any isotope with a high cross sectionfor thermal neutrons including but not limited to lithium-6 and thelike. The detecting layer is formed in some embodiments with asilicon-based thin film PIN or NIP diode structure/detector, and, insome cases, the thin film includes amorphous silicon (i.e., a-Si:H). Thedetector could also be of a-SiC:H or nanocrystalline silicon or anotherwide bandgap thin film semiconductor material. To facilitate sensitivedetection of current spikes in the detecting layer, embodiments of theneutron detector include a pixilated top electrode or contact (e.g., aplurality of smaller top contacts and, it should be remembered thatpixilation may also be at the bottom electrode or contact) near thedetecting layer rather than a single large top contact layer astypically found in solid-state devices. Further, the pixilation size andshape was chosen in some cases to control capacitance (e.g., matchcapacitance requirements to a connected detection device such as apreamplifier circuit/chip) and also to position an edge of each topcontact pixel or pixilated contact near or at the edge of the detector(or a supporting substrate) to facilitate electrical connection (e.g.,wiring to an amplifier and/or connection to a ribbon connector).Efficiencies may be increased in some embodiments of neutron detectiondevices by providing multiple layers or a stack of individual neutrondetectors. For example, a single layer neutron detector may have anefficiency of about 3 percent or less, but a neutron detection devicewith an assembly or stack of two or more neutron detectors may be ableto increase the efficiency of the detection device to 10 to 30 percentby including four to ten or more thin film neutron detectors.

FIG. 1 illustrates schematically a thin film neutron detection device100 that may be used to provide high sensitivity detection of neutrons(e.g., presence of plutonium or other radioactive material emittingthermal neutrons). The detection device 100 includes a substrate 110such as a thin layer of plastic, glass, ceramic, metal, or the like. Athin film silicon-based detecting layer 120 is formed or provided uponthe substrate 110. In some embodiments, the layer 120 takes the form ofan a-Si:H p-i-n (or PIN) or n-i-p (NIP) diode structure with athickness, detector, of less than about 30 microns and more typicallyless than about 10 microns. The detection device 100 further includes aB-10 neutron conversion layer (or thin film) 130, which may be a layerwith a thickness, t_(B-10 layer), of less than about 5 microns, moretypically less than about 4 microns, and even more typically less thanabout 2 microns. The B-10 neutron conversion layer 130 may be boron,boron carbide (B₄C), boron nitride (B₃N₃), nano- or micro-particlescontaining B-10 or other materials in an organic filler material, or thelike enriched with or including the isotope boron-10. A chip-basedamplifier or detection amp 154 is electrically connected to the device100 by leads 150, and a power source or battery 156 may be included toprovide reverse bias to the pin diode and enhance detection, and theleads 150 typically would be connected to a bottom contact layer orelectrode (not shown in FIG. 1) and to pixilated top contacts orelectrodes (not shown in FIG. 1 but described in detail at least withreference to FIGS. 2 and 3). The detection amplifier could be includedas a thin film device deposited on the same substrate as the p-i-n orn-i-p sensing diode.

In operation, neutrons 160 (e.g., slow or thermal neutrons) strike theneutron conversion layer 130. In response, the B-10 particles orcomponent of the layer 130 emit alpha particles that are transmittedthrough portions of the conversion layer 130 and into the detectinglayer 120. The alpha particles 170 are detected in the detecting layer120 via electron-hole pairs 180 and the chip-based amplifier 154, whichsenses pulses or current spikes associated with each or nearly eachalpha particle 170 that is transmitted into the detecting layer 120.

FIG. 2 illustrates a sectional view of one embodiment of a neutrondetector or detector sheet 200. The detector 200 includes a thin (e.g.,less than about 0.5 mm or the like) substrate 210 of glass, plastic,ceramic, metal, or other material useful for thin film substrates. Aneutron conversion layer 220 (e.g., B-10 or its compounds such as B-10carbide, B-10 nitride, and the like) is coated upon the substrate 210,and a metal bottom electrode or contact layer 230 is deposited over allor a portion of the conversion layer 220. A silicon-based detectinglayer 240 is coated over the bottom electrode 230, and the layer 240 maycomprise amorphous or other thin-film silicon with a thickness in somecases of less than about 10 microns. The layer 240 may be formed toprovide an a-Si:H PIN or NIP diode structure. On an upper surface 242 ofthe detecting layer, a pixilated top electrode 250 is provided fordetecting when alpha particles emitted by the neutron conversion layer220 (in response to thermal neutrons striking the conversion layer 220)transit the detecting layer 240.

In practice, the signals produced by the layer 240 (e.g., by about100,000 e⁻-h⁺ pairs when an alpha particle is stopped in a-Si:H materialof layer 240) are transmitted via the contact pixels 250 or pixilatedcontact 250 to a connected or linked detection circuit/chip (such as anexternal chip-based amplifier linked to each pixel of contact 250). Insome embodiments, the neutron detector 200 may include the layers 220,230, 240, and 250 on the other side of the substrate, as a detector 200may include the detection layers on either or both sides of thesubstrate 210 and also may include two or more of the neutron conversionlayers 220 and a like or differing number of the detecting layers 240.Also, the neutron conversion layer 220 may be underneath the detectinglayer 240 and bottom electrode 230 as shown in FIG. 2, or be on top ofdetecting layer 240 and top pixilated contact 250, on both sides of thesequence comprising the detecting layer 240 and its top and bottomcontact layers 230, 250. In other cases, the neutron conversion layer220 may be separated from the substrate 210 with the sequence comprisingthe detecting layer 240 and its top and bottom contact layers 230, 250by an air gap (or thin layer of material that allows passage of at leasta portion of the alpha particles from the conversion layer 220).

As shown in FIG. 3, the pixilated contact or top electrode 250 is shapedand oriented so as to allow electrical contacting at the edge 244 of thedetector element as shown with connections 312 to leads 310 (whichtypically would be attached to an amplifier or similar device with someembodiments providing such an amplifier or device provided for eachpixel of contact 250). The neutron detector device 200 includes apixilated top electrode 250 that includes a plurality of long stripsthat extend from a first end 252 near the substrate/device edge 244 to asecond end 254 at or proximate to the opposite side/edge of thesubstrate 240. The elongated bodies or strips of electrode 250 may insome cases have a length, L_(electrode), of up to a few centimeterswhile having a width, W_(electrode), that is much smaller such as lessthan a few millimeters. The electrode pixels are shown to be spacedapart on surface 242 and typically will be substantially parallel (e.g.,their longitudinal axes are all substantially parallel). The electrode250 or its pixels may extend nearly across the detector 200 (e.g.,L_(electrode) may be about equal to the width, W_(detector)) or it maybe a substantial fraction such as at least about 70 percent, but in somecases such as shown in FIG. 7, pixels of the electrode 250 may extendfrom opposite edges to provide coverage of the substrate surface 242. Inuse, each pixel of conductor 250 is capable, via an adjacent orconnected electronic circuit or chip such as an amplifier as shown inFIG. 7, of detecting current spikes from the a-Si:H detecting layer 240due to single alpha particles produced by single neutron absorptionevents in the neutron conversion layer 230.

To increase or maintain the sensitivity of the detector 200, it isuseful to keep the pixels of electrode 250 small (in thickness andsurface area) to reduce the electrical capacitance of the system andrender the current spikes from a single alpha particle detectable (e.g.,with TFT or other amplifiers either on the detector 200 or external toit). To this end, the surface area of each pixel may be maintained belowabout 10 cm², and, in one embodiment, the surface area is maintainedless than about 0.5 cm² to match the capacitance of each pixel tooperating parameters of a particular preamplifier that may be connectedto a pixel of the top contact or electrode 250. It is also useful insome cases to cover as large a fraction as possible of the surfacewithout exceeding the capacitance limit on each pixel. Hence, the gapsbetween pixels may be kept relatively small compared with their width.The width, W_(electrode), of the pixels of the electrode 250 may beselected to achieve a particular surface area based on the pixel length,L_(electrode) (e.g., length chosen to nearly match detector width,W_(detector), or to extend across a fraction of the detector such asacross half of the detector as shown in FIG. 7). It may also bedesirable to keep the resistances of both the bottom electrode 220 andthe top electrode 250 relatively low in order to reduce the timeconstant. The metal electrode layer or layers that lie between theneutron conversion layer and the PIN or NIP diode detector layer (e.g.,an electrode adjacent to the conversion layer 420 such as electrode 430in the detector 400 shown in FIG. 4) should typically be thin enough(e.g., thinner than about 100 nanometers) in order to pass the alphaparticles generated in the neutron conversion layer without significantloss of their energy to the detecting layer. Additionally, for theabovementioned metal contact layer to effectively pass the alphaparticles, the electrode(s) may be microstructured to be able to performthis function (e.g., one or both of the electrodes of a detector 200,400, and so on may include micron-size, 10 micron-size, 100 micron-size,or the like size holes).

FIG. 4 illustrates a thin film neutron detector 400 similar to that ofFIGS. 2 and 3 with the neutron conversion layer 420 deposited directlyupon the substrate 410. The a-Si:H PIN diode detector 440 along withbottom contact layer 430 and top metal contact 450 (which is provided onsurface 442) may be attached to the conversion layer 420 (or built up onthe substrate 410 and conversion layer 420 by thin film depositionprocesses). In this manner, the conversion layer 420 and a-Si:H PINdiode structure 440 may be fabricated in separate steps and assembled ata later time to form detector 400. The metal contact layer 430 may abutthe conversion layer 420, and the contact layer 430 may be madesufficiently thin or microstructured to facilitate passage of alphaparticles from conversion layer 420 to detecting layer 440, or theconversion layer 420 may be on the top of top electrode 450, abut withthe latter or even be separated by an air gap. In one example, thesubstrate 410 may be a plastic foil and the neutron conversion layer 420may be sprayed or inkjet printed upon the substrate 420 via inkcontaining nanoparticles or the like providing the B-10 while the PINdiode structure 440 may be formed with known process for use withamorphous silicon formation.

It is believed that large arrays of amorphous silicon PIN diodes can befabricated inexpensively, which will facilitate fabrication of detectors400 which may be combined to form large area detectors (e.g., pluralityof small detectors 400 may be arranged side-by-side to cover a largerarea such as square meters in size). The per unit area cost will likelybe as little as ten percent of the cost of Helium-3 tube neutrondetectors. The semiconductor neutron detector 400 can be fabricated tobe very thin, e.g., less than about 1 millimeter, while being asefficient as Helium-3 tube neutron detectors and other detectors.Additionally, it is believed that the detectors 400 will require littlepower to operate and will not require high voltage. Thin neutrondetection devices including the detectors 200 and/or 400 may be deployedin many settings where large and bulky Helium-3 tube neutron detectorswould be impractical.

In one embodiment, manufacture of the detector 400 includes depositingthe B-10 conversion layer 420 upon a glass substrate 410, and thenapplying the metal contact layer 430 using chromium deposition by e-beamor thermal evaporation. The a-Si:H PIN or NIP diode structure 440 isprovided using plasma enhanced chemical vapor deposition or other meansof depositing high-quality hydrogenated amorphous silicon or relatedmaterials such as hot-wire chemical vapor deposition or reactivesputtering such as by applying the n-layer (e.g., 30 nanometers (PH₃doped a-Si:H), the i-layer (e.g., 2.4 micrometers intrinsic a-Si:H),applying a graded buffer layer (e.g., approximately 50 angstroms thick),and applying the p-layer (e.g., 10 nanometers of B-doped a-SiC_(x):H).The pixilated top contact 450 is then provided via mask.

The thin layers of boron-10, lithium-6, or the like utilized to form theneutron detectors described herein (e.g., 2 micron thick layers ofmaterials containing boron-10, lithium-6, or other isotopes) may haverelatively low neutron conversion efficiencies. For example, a 2-micronthick neutron conversion layer may only stop about 3 percent or less ofthe thermal neutrons that strike a detector. The thickness, though,typically is limited in an attempt to ensure that alpha particlescreated in collisions are successfully emitted from the neutronconversion layer for detection within an a-Si:H detecting layer, andalpha particles are relatively easily shielded (e.g., may be stopped bya few microns of material in the neutron conversion layer). Hence, itmay not be useful to simply thicken the neutron conversion layer.Instead, embodiments of detectors may include additional neutronconversion layers, such as by providing neutron conversion layers anddetecting/contact layers on both sides of a substrate. In this manner,the efficiency of the detection device may be doubled to 5 to 6 percentor the like.

For applications demanding an efficiency greater that about 6 percent,it may be useful to stack or layer 3 to 10 or more individual neutrondetectors or sheets/layers. For example, there are some applicationswhere detection needs to be relatively quick because the monitoredobject (e.g., a shipping container, luggage, or the like) is moving pastthe detector and/or the background noise and/or likely attempts atshielding require high sensitivity. FIG. 5 illustrates a neutrondetection device 500 that may include three or more neutron detectors510, 530, 550. In one example, the device 500 includes ten detectors510, 530, 550 such that the efficiency ranges between about 25 and 30percent. The detectors or detector sheets 510, 530, 550 are stacked intoa multilayer structure or device 500 for high sensitivity, and thedevice 500 may be fabricated by mechanically assembling individualdetectors or by repeated deposition of thin film device layers usingpatterning techniques such as masking, photolithography, or the like.Pixel connections extending to the side of the substrate allow for easystacking of elements and electrical connection, e.g., into a socket tothe circuitry (not shown) that processes the electronic pulse readoutfrom each pixel or portions of pixilated contact/electrode.

In the device 500, the three neutron detectors 510, 530, 550 areconfigured with matching or identical arrangement of thin film layers,but, in other embodiments, the detectors 510, 530, 550 may differ fromeach other (e.g., take any of the forms described herein). As shown,each neutron detector or detector sheet 510, 530, 550 includes asubstrate 512, 532, 552, a metal contact layer 514, 534, 554, and a PINdiode detector (e.g., an amorphous silicon thin film fabricated toprovide electron-hole pairs to detect alpha particles) 516, 536, 556. Inthis embodiment, a set of elongated top contacts or pixels 518, 538, 558are provided upon the PIN diode detectors 516, 536, 556, and the B-10layer, lithium-6 layer, or neutron conversion layer 520, 540, 560 isprovided on the contacts or pixels 518, 538, 558 (e.g., the neutronconversion material is pixilated or formed of elongated bodies or thinstrips with dimensions similar to that of the supporting pixilatedcontacts). Thermal neutrons striking the upper detector 550 will eithercollide with a boron-10 isotope generating alpha particles (about 3percent of the neutrons) or be passed to a next or lower neutrondetector (with this process repeated throughout the neutron detectiondevice 500). As with the other detector embodiments, the capacitances ofeach of the contact pixels of contacts 518, 538, 558 may be matched to aconnected amplifier or pulse detection device, such as by limiting thesurface area of each pixel to less than about 1 cm² and in some casesthe surface area is about 0.5 cm² for each contact pixel (on oneprototype, for example, each pixel had a length of about 24 millimetersand a width of about 3 millimeters).

The use of long and thin strips for the top contacts (e.g., a pixilatedcontact layer) allows the contacts to extend to the detector edge makingwiring extremely easy (e.g., fitting into a socket of a ribbon cable orthe like). Additional functionality may also be provided in a neutrondetector using this contacting scheme. For example, FIG. 6 illustrates atwo-layered neutron detection device 600 that not only provides enhancedefficiency (e.g., twice the efficiency of a one-layer structure) butalso provides (or facilitates) imaging or location determination (e.g.,locations of where neutrons are striking the neutron conversion layer).As shown, the device 600 includes a first detector 610 and a seconddetector 620 positioned over or stacked upon the first detector 610. Thedetectors 610, 620 may take the form of any of the detectors shownherein. The first detector 610 includes an upper substrate (e.g., ana-Si:H PIN or NIP diode structure or neutron conversion layer) 612 uponwhich a plurality of contact pixels or a pixilated contact 614 isprovided. The elongated pixels 614 are shown to extend from a first edgewhere they are connected to leads 618. The second detector 620 likewiseincludes an upper substrate 622 and pixels 624 extend from an edge wherethey are connected to leads 628, and the leads 618, 628 would beconnected to amplifiers or other devices for detecting current spikesassociated with neutron conversion into alpha particles by the detectors610, 620.

As shown, the detectors 610, 620 are stacked together such that they arerotated at angular offset such that the pixels 614, 624 are notparallel. More particularly, the pixels 614, 624 are orthogonal to eachother (e.g., top detector 620 is rotated about 90 degrees relative tothe detector 610). In an operating example, area 640 may be associatedwith a higher neutron flux. Neutrons penetrate the layers of thedetectors 610, 620 within area 640 and produce alpha particles in therespective boron-10 or conversion layers adjacent amorphous silicondetecting layers. This causes signals to be detected on the pixelsassociated with leads B₁ (first layer 610) and C₂ (second layer 620). Byprocessing the signals (with hardware and/or software associated withamplifiers or similar devices connected to leads 618, 628 but not shownin this example), the signal intensities in each pixel can be determinedand the image may be constructed (e.g., by generating a matrix fordetected signals in each pixel and/or each intersection of two pixels inadjacent layers 610, 620).

In this example, such imaging would indicate the area 640 of thedetection device 600 is a location receiving a relatively high neutronflux and such location on a detection device 600 may be used to identifya location of a neutron source (e.g., a volume of plutonium or otherradioactive material) such as within a container of material or objects.The stacked detectors 610, 620 or a similar configuration may also beused in medical neutron imaging with B-10, lithium-6, or other isotopesutilized to act to block or as a block for neutrons. A neutron sourcewould e targeted at parts of the patient's body to image the locationsof 10-B, lithium-6, or other neutron-blocking isotopes in, on, or nearthe patient's body.

FIG. 7 illustrates a representative neutron detection device 700, with ahousing removed or not shown. The device 700 includes a board 710 uponwhich a neutron detector 720 is provided. The neutron detector 720 isshown to include a substrate 722 (such as a thin-film hydrogenatedsilicon p-i-n or n-i-p diode detector) and a plurality of top contacts724 or a pixilated contact layer is deposited upon the substrate 722.The contact layer differs in that each pixel or contact portion 724 onlyextends about half way across the width of the substrate 722 but pairsof pixels 724 extend across the substrate 722 with ends connected vialeads 726 to adjacent or nearby contacts or lead pads 728 on board 710.The contact pads 728 are then electrically connected to correspondingpulse/spike detection amps or circuits 730 (e.g., low-noise, FET preampused to condition PIN or NIP diode output or the like), which functionto detect spikes in current in the silicon-based detecting layer of thedetector 720.

While not limiting, one implementation of the device 700 had a detector720 with an overall size of about 2.54 cm by about 3.81 cm (e.g., 1-inchby 1.5 inches). The detector 720 had 22 rectangular contact pixels, with20 being active and 2 being used to contact the bottom electrode. Thepixel size was 2.8 mm by 11.5 mm, resulting in each pixel having a pixelsurface area of about 0.32 cm² (or less than about 0.5 cm²). Thedetector 720 had the following thin-film layers on top of a glasssubstrate: a neutron conversion layer including boron-10 carbide atabout 1 to 1.5 micron thickness deposited, for example, by sputtering; abottom metal contact layer formed of chromium; a thin-film amorphoushydrogenated silicon p-i-n diode detector with a total thickness ofabout 2.4 microns; and a pixilated top contact layer formed ofpalladium. The bottom metal contact may be made thin enough (e.g., 50 to100 nanometers thick) to allow alpha particles generated in the neutronconversion layer to pass through the bottom metal contact and enter thep-i-n or n-i-p silicon detector layer without losing a significantfraction of their energy.

This implementation was successfully tested for neutron detection asfollows. A 252Cf source with 1-inch high-density polyethylene was usedas the source of slow neutrons. Each of 20 detector pixels 724 wasconnected to a low noise field-effect-transistor (FET) preamplifier usedto condition PIN diode output. The neutron irradiation test resultsshowed that the pixels 724 of neutron detection device 700 wereapproximately 2.5 percent efficient in detecting the incident neutrons.Sensitivity of the thin-film neutron detector pixels 724 wassimultaneously compared to a 3He neutron tube with known sensitivity.The PIN diodes were also subjected to a strong gamma flux, and no gammaresponse could be measured over several hours time. This demonstratesthat the implementation is capable of detecting neutrons at efficienciesbetween 2 and 3 percent, with excellent discrimination between incidentneutrons and gamma particles.

Sputtering and similar methods may be used to deposit the neutronconversion layer in the neutron detectors. However, in some cases,sputtering may lead to cracking and peeling problems as well assometimes resulting in poor adhesion and uneven deposition of boron(e.g., when boron, boron carbide, or boron nitride is sputtered upon asubstrate or onto an a-Si:H PIN or NIP diode structure). In someembodiments, it may be useful to deposit the neutron conversionlayer/material in another way such as when the neutron conversion layer(or B-10 material, lithium-6, or the like) is deposited over thepixilated top contact. In such embodiments, it is important for the topcoating with the B-10 isotopes to only cover the electrode or contactpixels and leave gaps in between adjacent pairs of such contact pixels.Otherwise, the top contacts may contact each other, and high fillfactors for the pixels can be achieved by using inkjet printing (e.g.,an ink or organic binder that includes nanoparticles of the boron-10enriched material such as ¹⁰B₄C, boron enriched with boron-10, or¹⁰B₃N₃), photoresist/liftoff processes, and the like. In some cases, thecontacts may be made of micro- or nano-particles in an organic binder sothat the coating of the boron-10 containing neutron conversion layer isinsulating and does not bring the top contacts into electrical contactwith each other. If the contacts are connected, a high reverse currentthrough the diode would result and the background noise associated withthis high reverse current would render the neutron-to-alpha currentspike more difficult or even impossible to detect.

For example, FIGS. 8A-8D show an alternative to inkjet printing of theboron-10 onto the substrate or onto pixilated contact. The process shownuses a combination of photolithography and spin-coating of B-10 ink asone useful process for forming a thin-film neutron detector thatcontrols/limits cracking/peeling of the neutron conversion layer. FIG.8A shows a neutron detector 800 after initial stages of fabrication havebeen performed including deposition of a bottom contact (not shown) anda thin-film PIN diode structure (e.g., a detecting layer) 820 have beendeposited or formed upon a substrate (e.g., a sheet/layer/foil of glass,plastic, metal, ceramic, or the like) 810. In FIG. 8A, a pixilated topcontact 830 is deposited or formed on the upper or exposed surface ofthe detecting layer 820. Next, in FIG. 8B, photoresist 840 is applied tofill the gaps between the adjacent metal contacts 830. In FIG. 8C, thedetector 800 is further fabricated by spin-coating with B-10 ink, andthe ink is cured (e.g., to remove solvents or the like and leave ¹⁰B₄C(or other boron-10 material) and one or more organic binders).

FIG. 8D shows the neutron detector 800 after liftoff of the photoresist830 as well as a portion of the B-10 material. As shown in FIG. 8D, thedetector 800 includes a B-10 neutron conversion layer 860 that is onlyapplied to the surfaces of the top contacts 830, and, as a result, theneutron conversion layer 860 is pixilated in a manner that matches thecontact layer 830 (e.g., a plurality of long, thin bodies/pixels orelongated strips of B-10 material with small gaps between parallelpixels). In some cases, the top contact metals and the B-10 layer may beinkjet printed so that there is no removal step to reach 860.Alternatively, the B-10 layer may be everywhere (as in 850) if it issufficiently insulating that it does not short the contacts 830.

FIG. 9 illustrates another embodiment of a neutron detection device 900that may be manufactured by separately fabricating a detecting assembly904 and a conversion assembly 908 and then laminating or otherwisecombining the two assemblies 904 and 908 with the conversion film 950 inabutting contact (such as when an insulating B-10 conversion film isused) or with an air gap provided between the two assemblies 904 and908. In the case of abutting contact, the neutron conversion layer maybe printed or otherwise deposited in a pattern that matches the PINelectrode pattern, with gaps in between, or is insulating enough toprevent contacting between the individual pixel electrodes of the PINdetecting layer. The detecting assembly 904 includes a substrate 910such as a thin layer or sheet of glass, metal, ceramic, or plastic (withplastic foil shown in FIG. 9). A bottom metal contact or electrode 920is deposited upon the substrate 910 and then an a-Si:H detector layer(such as a PIN diode structure) 930 is formed upon the contact 920 (suchas using the vacuum deposition or other thin-film processes discussedabove). Next, a pixilated top electrode 940 is formed upon the detectinglayer 930, such as a plurality of elongated or thin rectangularpalladium pixels with relatively small capacitance (e.g., by maintainingthe thickness and surface area relatively small such as a surface areaof less than about 0.5 cm²).

The conversion assembly 908 may be formed by spraying, spin-coating,inkjet or other printing, or other deposition techniques (includingsputtering and other thin-film processes). In one case, a substrate 960such as a flexible plastic foil is provided and a volume of inkincluding B-10 particles and organic binders are sprayed or printed uponthe foil 960. The ink is then cured to produce the B-10 neutronconversion film 950, which may be a large percentage of B-10 nano- ormicroparticles and a small percentage of organic binder. The conversionfilm 950 typically is relatively thin with a thickness of less thanabout 5 microns and more typically less than about 2 microns (with someprototypes using thickness of about 1 to 1.5 microns with the particlesbeing ¹⁰B₄C or the like). The conversion assembly 908 may then belaminated or otherwise layered on or near the detecting assembly 904 asshown with arrows 970. In other embodiments, a second conversionassembly 908 is applied by lamination or other assembly techniques tothe other or opposite side of the substrate 910 to increase thesensitivity of the detection device 900. In still other embodiments, aplurality of the conversion and detecting assembly pairs 904, 908 arecombined in a stacked arrangement to achieve a desired sensitivity ofdetection device 900 (such as 10 to 30 percent or more).

With the above description in mind, it will be understood that theneutron detectors taught provide an all-thin-film device that addressesprior problems associated with crystalline-based solid-state devices.The thin-film neutron detectors and neutron detection devices fabricatedwith such detectors may be produced at a fraction of the cost ofexisting neutron detectors and prototyped crystalline devices.Additionally, the detectors may be combined in a 2D pattern to produce aneutron detection device with a larger surface area (e.g., up to squaremeters in area). The detectors or devices may be provided on flexiblesubstrates, and they also may be fabricated to provide a relativelylightweight finished product.

The above description also presents several unique ways of producing theB-10 neutron conversion layer (e.g., the B-10 carbide, B-10 nitride,boron, or similar thin film) such as by using sputtering or other thinfilm processes and based on use of nanoparticle inks. The neutronconversion layer may be manufactured, as discussed above with referenceto FIGS. 8A-9, with B-10-containing ink. Such ink of B-10 (or B-10compounds such as nitride or carbide) can be based, for example, onnano- or microparticles of these compounds. The ink can be spin coatedin conjunction with photoresist masking and liftoff. In other cases,though, the ink may be inkjet printed onto a substrate or other layer ofthe detector in a desired pattern. This latter technique may beespecially useful for producing the neutron conversion layer on top ofthe top electrode pattern (e.g., on top of a plurality of elongatedcontact pixels) with the gaps or spaces between the contact or electrodepixels not being filled with neutron conversion materials. In othercases, the sprayed or spin-coated conversion layer can be sufficientlyinsulating so that the multiplicity of detector pixels remainelectrically isolated from one another.

In addition it may be useful to stress several other useful aspects orfeatures of the described neutron detectors and neutron detectiondevices formed with such detectors. In some embodiments, it isbeneficial to provide two B-10 containing layers in a detector. Forexample, a B-10 containing layer may be provided on both the bottom andtop of the thin-film diode detector (e.g., a-Si:H-based detecting layer)that acts to detect alpha particles produced by neutrons received by orstriking the B-10 particles in the two layers sandwiching the detectinglayer. The two B-10 containing layers (bottom and top or first andsecond, spaced apart layers) may be produced by the same or differingdeposition methods and may have similar or differing compositions andphysical configurations (e.g., one may be sputtered onto a substrate orcontact layer and be formed of a B-10 compound such as nitride orcarbide while the other is applied using inkjet printing of an inkcontaining nanoparticles with B-10).

The thin-film detectors also lend themselves to being stacked in one ofseveral ways to increase overall sensitivity of the finished device(e.g., the devices shown in FIGS. 5 and 6 and so on). One way to providea stacked detection device is to deposit a second (and consequently, thethird, fourth, and so on) detector directly on top of the previousdetector using thin film deposition processes with appropriate maskingtechniques. Here, the B-10 containing layers serve as neutron conversionlayers for PIN diode or other detecting layers on both sides of theseB-10 neutron conversion layers (e.g., top and bottom conversion layerssandwich a-Si:H detecting layers and the conversion layers emit alphaparticles in all directions and not just the direction of the path ofthe received or striking neutron). In this manner, it is not justdeposition of many individual detectors on top of each other, but thedepositing or fabrication of an integrated, multilayer detection devicethat contains several neutron conversion layers and several detectinglayers (e.g., PIN diode alpha detectors).

Another method of forming a stacked arrangement involves depositingindividual detectors (e.g., diode with neutron conversion layer(s)) onthin (and, optionally, flexible) substrate sheets, with a detectorprovided upon one or both sides of the substrate sheet. Then,fabrication of the device involves laminating or otherwise assemblingtwo or more of the substrate sheets together into one, still relativelythin (and, optionally, flexible) multilayer detection sheet or device.In this design, additional neutron conversion layers can also bedeposited separately on similar substrate sheets and laminated togetherwith diode detectors on different sheets, as an alternative to directdeposition of the top neutron conversion layers.

The invention claimed is:
 1. An apparatus for detecting neutrons, comprising: an alpha particle-detecting layer comprising a thin film of silicon material; and a neutron conversion layer comprising a material including isotopes emitting alpha particles to the alpha particle-detecting layer when neutrons strike the neutron conversion layer, wherein the alpha particle-detecting layer comprises an amorphous silicon p-i-n or n-i-p diode structure, wherein the apparatus further comprises a bottom contact layer and a top contact layer, wherein the amorphous silicon diode structure is positioned between the bottom and top contact layers, wherein the top contact layer comprises a pixilated metal layer comprising a plurality of contact pixels extending proximate to an edge of the apparatus, and wherein the neutron conversion layer is deposited upon the contact pixels of the top contact layer, the neutron conversion layer comprising a plurality of elongated conversion pixels and the top contact layer being sandwiched between the neutron conversion layer and the alpha particle-detecting layer.
 2. The apparatus of claim 1, wherein the isotopes comprise boron-10 or lithium-6 isotopes.
 3. The apparatus of claim 1, wherein each of the contact pixels comprises an elongated body with a surface area of less than about 1 cm².
 4. The apparatus of claim 1, further comprising a substrate and wherein the neutron conversion layer has a thickness of less than about 2 microns and is positioned between the substrate and the alpha particle-detecting layer.
 5. A neutron detection device, comprising: a first thin-film neutron detector; and a second thin-film neutron detector positioned adjacent to and at least partially overlapping the first thin-film neutron detector, wherein each of the neutron detectors comprises a neutron conversion layer containing isotopes emitting alpha particles when struck by neutrons and further comprises a top and a bottom metal electrode with an amorphous silicon diode detecting layer sandwiched between the top and bottom electrodes, wherein the neutron conversion layer of each of the neutron detectors is positioned abutting the top metal electrode or the bottom metal electrode on a side opposite to the amorphous silicon diode detecting layer, and wherein the neutron conversion layers of the first and second thin-film neutron detectors are spaced apart from each other by at least one of the amorphous silicon diode detecting layers.
 6. The device of claim 5, further comprising at least a third, a fourth, and a fifth thin-film neutron detector and each of the neutron detectors is arranged in a stacked configuration and the neutron detection device having a sensitivity of at least about 10 percent.
 7. The device of claim 5, wherein the neutron conversion layer has a thickness of less than about 2 microns and the amorphous silicon diode detecting layer has a thickness of less than about 10 microns to control gamma reactions.
 8. The device of claim 7, wherein the second neutron detector is supported upon a flexible substrate and the second neutron detector is stacked upon the first neutron detector by laminating the flexible substrate to the first neutron detector.
 9. The device of claim 5, wherein each of the top electrodes comprises a pixilated layer of metal comprising a plurality of pixels, wherein each of the pixels comprises an elongated body with an end extending to an edge of the neutron detector, and wherein the isotopes comprise boron-10 or lithium-6 isotopes.
 10. The device of claim 9, wherein the first and second neutron detectors are positioned relative to each other such that a longitudinal axis of the pixels in the first detector is substantially orthogonal to a longitudinal axis of the pixels in the second detector to facilitate imaging of detected neutrons.
 11. An apparatus for detecting neutrons, comprising: an alpha particle-detecting layer comprising a thin film of silicon material; a neutron conversion layer comprising a material including isotopes emitting alpha particles to the alpha particle-detecting layer when neutrons strike the neutron conversion layer; and a bottom contact layer and a top contact layer, wherein the top contact layer comprises a pixilated metal layer comprising a plurality of contact pixels extending proximate to an edge of the apparatus, and wherein the neutron conversion layer is deposited upon the contact pixel of the top contact layer, whereby the neutron conversion layer comprises a plurality of elongated conversion pixels and whereby the top contact layer is positioned between the neutron conversion layer and the alpha particle-detecting layer.
 12. The apparatus of claim 11, wherein the alpha particle-detecting layer comprises an amorphous silicon p-i-n or n-i-p diode structure.
 13. The apparatus of claim 12, wherein the amorphous silicon diode structure is positioned between the bottom and top contact layers.
 14. The apparatus of claim 11, wherein the contact pixel comprises an elongated body with a surface area of less than about 1 cm².
 15. The apparatus of claim 11, further comprising a substrate and wherein the neutron conversion layer has a thickness of less than about 2 microns and is positioned between the substrate and the alpha particle-detecting layer.
 16. The apparatus of claim 11, further comprising a second neutron conversion layer.
 17. The apparatus of claim 16, wherein the alpha particle-detecting layer is sandwiched between the two neutron conversion layers. 